Method of stabilizing single channel analyzers

ABSTRACT

A method and the apparatus to practice the method in which the drift of single channel analyzers is reduced. Essentially, this invention employs a time-sharing or multiplexing technique to insure that the outputs from two single channel analyzers (SCAS) maintain the same count ratio regardless of variations in the threshold voltage source or voltage changes. the multiplexing technique is accomplished when a flip flop, actuated by a clock, changes state to switch the output from the individual SCAS before these outputs are sent to a ratio counting scalar. In the particular system embodiment disclosed that illustrates this invention, the sulfur content of coal is determined by subjectiong the coal to radiation from a neturon producing source. A photomultiplier and detector system equates the transmitted gamma radiation to an analog voltage signal and sends the same signal, after amplification, to a SCA system that contains our invention. Therein, at least two single channel analyzers scan the analog signal over different parts of a spectral region. The two outputs may then be sent to a digital multiplexer so that the outputs may then be sent to a digital multiplexer so that the output from the multplexer contains counts falling within two distinct segments of the region. By dividing the counts from the multiplexer by each other, the precentage of sulfur within the coal sample under observation may be determined.

United States Patent Fasching et a1.

[451 Feb. 25, 1975 1 1 METHOD OF STABILIZING SINGLE CHANNEL ANALYZERS[57] ABSTRACT A method and the apparatus to practice the method in whichthe drift of single channel analyzers is reduced. Essentially, thisinvention employs a time-sharing or multiplexing technique to insurethat the outputs from two single channel analyzers (SCAS) maintain thesame count ratio regardless of variations in the threshold voltagesource or voltage changes. the multiplexing technique is accomplishedwhen a flip flop, actuated by a clock, changes state to switch theoutput from the individual SCAS before these outputs are sent to a ratiocounting scalar.

1n the particular system embodiment disclosed that illustrates thisinvention, the sulfur content of coal is determined by subjectiong thecoal to radiation from a neturon producing source. A photomultiplier anddetector system equates the transmitted gamma radiation to an analogvoltage signal and sends the same signal, after amplification, to a SCAsystem that contains our invention. Therein, at least two single channelanalyzers scan the analog signal over different parts of a spectralregion. The two outputs may then be sent to a digital multiplexer sothat the outputs may then be sent to a digital multiplexer so that theoutput from the multplexer contains counts falling within two distinctsegments of the region. By dividing the counts from the multiplexer byeach other, the precentage of sulfur within the coal sample underobservation may be determined.

8 Claims, 4 Drawing Figures AMPLIFIER C MULTIPLEXED SINGLE CHANNELANALYZER SYSTEM SYSTEM [75] Inventors: George E. Fasching; George H.

Patton, both of Morgantown, W. Va.

[73] Assignee: The United States of America as represented by theSecretary of the Interior [22] Filed: Dec. 5, 1973 [21] Appl. No.:422,052

[52] US. Cl 250/336, 250/358, 250/369 [51] Int. Cl. G01t 1/20 [58] Fieldof Search ..328/105,115,116,117; 250/253, 262, 263, 358, 336, 369

[56] References Cited UNITED STATES PATENTS 2,648,012 8/1953Scherbatskoy 327/115 3,105,149 9/1963 Guitton eta1.... 250/369 3,270,2058/1966 Ladd et a1 328/117 3,509,341 4/1970 Hindel et a1. 328/1173,532,977 10/1970 Giordano eta1.... 328/117 3,747,001 7/1973 Fasching eta1 328/116 3,752,984 8/1973 Scott et a1 250/262 Primary Examiner-HaroldA. Dixon Attorney, Agent, or FirmThomas Zack DETECTOR B ANDPHOTOMULTIPLIER H.V. POWER RATIO COUNTING SCALARS PATENTEDFEB 2 5 I975SHEET 2 0f 3 SCA I R2 RI T-RC] UPPER THRESHOLD SCA 1I LOWER THRESHOLDUPPER THRESHOLD RH RIO FIG. 2.

LOWER THRESHOLD CLOCK DRIVER PMENIEIIFEB25 I975 3 5 5 39 SHEET 3 BF 3IIY .3,7oo- Z 0 5 i .I/SULFUR PEAK U) o E I 8 I I '0 0 0 o SEGMENT 2l,800 i= T I I l 2- I SEGMENT I I II I I l l I I I CHANNEL NUMBER Y=4076.5 -e.1557sx 5 3,700

\ U) 2 8 TANGENT I,aoo ---I E I LINE I I l I i fl I i: i SEGMENT l g I II I "X CHANNEL NUMBER METHOD OF STABILIZING SINGLE CHANNEL ANALYZERSBACKGROUND OF THE INVENTION 1. Field of the Invention The method andapparatus to practice the method described herein relates to astabilization technique for two single channel analyzers. Morespecifically, this invention stabilizes the relative threshold levels ofsingle channel analyzers by a time sharing technique such that theircount ratio remains constant.

2. Description of the Prior Art Stabilization of the output signals fromsingle channel analyzers is a problem caused by variations in thecircuit components and voltage drifts in the reference source networksand comparators. One solution has been to use multichannel analyzers inplace of the single channel analyzers in a ratio counting system.However, because of the cost, slowness, and complexity of these units,the results have been limited. Phase differences between two signalshave been analyzed in such patents as the Miller reference having USPat. No. 3,63 l ,340. In none of the known prior art references is thevoltage amplitude discrimination of random pulses that is being sent totwo single channel analyzers stabilized by using a time-sharingtechnique.

SUMMARY In our invention, the same analog voltage is fed to two singlechannel analyzers operating between two different upper and loweroperating ranges. The discriminator thresholds and output counts fromthe analyzers are switched by a digital multiplexer such that the outputpulse counts from one range always appear at the same output terminal.Next, a cpmparison is made of the counts from the outputs of the tworanges to determine a desired result.

The primary object of our invention is an improved method and apparatusto stabilize the outputs from two single channel analyzers.

A secondary object of our invention is an improved detecting system todetect specific components of a material under observation.

FIG. 1 is a block diagram of the system set up.

FIG. 2 is a circuit diagram of the multiplexer system.

FIG. 3 is a graph of detected sulfur and other peaks based on the countsper minute detected.

FIG. 4 is a graph similar to the FIG. 3 graph showing a graphicalrepresentation of a formula and its relation to the sulfur peaks.

The system set up of FIG. I shows an actual working embodimentincorporating the principles of our invention. A neutron producingsubstance 1 sends its rays into the material under observation 5 that isheld in a hopper 3. The radioactive source element selected wasCalifornium (Cf) 252 and the material under observation was coal. Asthese neutrons strike the coal atoms, gamma rays from the coal atomsnuclei are produced and transmitted as a natural occurring phenomena tothe ambient air. What this experiment sought to determine was the sulfurcontent of the coal. After the rays pass through the coal and hopper, aphoto-multiplier detector powered from a high voltage (HV) source, viasignal A, detects the transmitted gamma wave portion and changes it intoan electrical representation. Next, this detected wave form is sent assignal B to an amplifier system where it is amplified electronically andemitted as signal C.

Signal C is an analog random voltage pulse whose amplitude can becorrelated to the energy of the detected gamma rays. It is sent to amultiplexed single channel analyzer (SCA) system shown as a single blockin FIG. 1. The essence of our invention resides in the performance andstructure of this multiplexing system and how it processes the analogsignal C.

When coal is subjected to neutron radiation, the peak height of signal Cis proportional to the energy of a detected gamma ray. When this signalenters the multiplexer system of FIGS. 1 and 2, it is sent to the twoSCAs I and II. These SCAs deliver a fixed pulse at their outputs ifsignal C falls within a spectral region or energy window individuallyfor each SCA. The SCA pulses X and Y are fixed in height and width attheir respective outputs but each SCA pulse is not necessarily in phasewith the other. Each SCA consists basically of a pair of discriminatorswhich send a signal to a comparator and certain pulse forming circuitry.The two output signals from these discriminators comparator, andassociated circuitry are sent to common logic circuitry to obtain thedesired fixed pulse that falls within a window or voltage range. TheseSCAs may be substantially identical in construction although such is notnecessary as long as the desired function of sending fixed pulses X andY is accomplished. The actual SCAs used by us were custom designed andbuilt by United States Bureau of Mines personnel. These SCAs would bevery similar to the SCA having Model No. 33-l4A built by RadiationInstrument Development Laboratory a division of Nuclear ChicagoCorporation.

Two reference voltage generator circuits are operatively connected toeach SCA to determine the voltage ranges or windows they will operatein. For example, the window for SCA l is set by adjustable resistors R RR and R For SCA II, adjustable resistors R R R and R would perform thesame function. Thus, the window for each SCA is defined upper and lowerthreshold limits. Normally these two windows are different from eachother and overlap each other as do the designated segments 1 and 2 isFIG. 3. In addition to the mentioned adjustable resistors, eachgenerator circuit has two fixed or supply voltage sources (El and E2; E3and E4), two fixed resistors RI and R4; R7 and R10), and two relay coilcontacts (RC1 and RC2; RC3 and RC4).

Continuing with the explanation of FIG. 2, the six NAND gates GI, G2,G3, G4, G5 and G6 form part of a digital multiplexer system whichreceive input gate drive signals A and A as well as, an inhibit signalN, and signals X and Y from SCA l and SCA II. The outputs from gates G5and G6 have been designated as signals D and E, respectively. Each ofthese outputs consist only of counts falling within a specific window.The four NAND gates G1, G2, G3, and G4 each have three input terminalsand one output terminal. These three input terminals may receive signalsfrom either one of the two SCAs from a multivibrator MV, or from drivergates A and A. Their single output terminals are connected to either ofthe NAND gates G5 or G6.

The remaining multiplexer system circuit elements of FIG. 2 consist of apulse clock connected to a flip flop FF] and a multivibrator MV, twodriver circuits A and A connected to FF], and two sets of relay coilsAl-A4 and A2-A3, with one set being connected to and operable by eachdriver circuit. The clock operates at a low frequency (about 1 pulse persecond) to send its output to FFl and the multivibrator MV. On eachcycle as the clock pulse is sent to the multivibrator, a 4 millisecondnoise gating signal N is outputted to the four NAND gates G1, G2, G3 andG4 to inhibit switching transients of PH and to allow circuit voltagesto settle. When the clock pulse triggers FFl, alternating comple mentarycontrol square waves are sent to drivers A and A. These drivers, whichare high output logic gates, in turn drive the relay coil pairs A1-A4and A2A3. Two relay coils are used for each driver to provide isolationfor switching at two different locations in the circuit for each set ofSCA discriminators. Relay contacts RC1 and RC2 of the reference voltagegenerators are actuated by relay coils A1 and A2 for SCA l. The contactsRC3 and RC4 are actuated by relay coils A3 and A4 for SCA ll.

When a periodic pulse is set by the clock to FFl and multivibrator MV achain of events is sent in motion. The pulse N exists for only 4millisecond which by experience has been shown as long enough to inhibitrelay bounce, which is always less than 4 milliseconds in duration inour set up. During this time interval, from to 4 milliseconds, all ofthe gates G1 to G4 (and hence gates G5 and G6) are inoperative eventhrough pulses are inputted on their other two input terminals. The flipflop circuit alternately drives A and A As it does this its connectedrelay coils actuate at least one contact in each reference voltagegenerator e.g., when driver A is actuated contacts RC2 and RC3 arecompleted. This results in pulses X and Y being set to gates G1, G2, G3and G4 from SCA land SCA 11 within their present window voltages. Thepart of these SCA pulses and the pulses A and A from driver A and A thatoccur after inhibit signal N has passed are what give output multiplexedpulses D and E from gates G5 and G6.

FIG. 3 is a graph of the coal under observation wherein the Y axisrepresents the counts per minute observed at the output and the X axisrepresents the channel number. This channel number corresponds to energyreceived in a pulse height analyzer having a sodium iodine (Nal) crystaland photomultie r tube which detects the gamma rays and converts them toelectrical pulses. Certain pulse amplitudes indicative of the energy ofthe sulfur that are referred to as sulfur peaks are observed on thegraph. Theoretically and empirically it can be shown that if the countcollected over a period of time from segment 1 are divided by two andthen divided by the counts in segment 2, a relation is found that can becorrelated to the percentage of total sulfur in the sample. While it isnot of critical importance to this invention on how the result wasformulated, the formula relating the output counts to each other wasdeveloped by United States Bureau of Mines personnel based both ontheoretical considerations and empirical work.

FIG. 4 is a graph very similar to the graph of FIG. 3 except that it isan approximation that has been slightly distorted to make the minimumsfor the four sulfur peaks such that they fall on a straight tangentline. The purpose of this graph is to allow a simplified comparison ofthe stability achieved by our invention with the state-of-the-artmethod. Since the purpose of our invention is to achieve relativestability at the outputs of the SCAs, the comparison can be made firstwhen the threshold supply voltage shifts and then when the window widthof one of the SCAs (segments 1 and 2 of FIGS. 3 and'4) change due to achange in the window supply voltage. Before, however, these two casesare looked at in detail, it should be noted that our improvement gainedby switching the SCAs not only applies to compensate for changes in thethreshold supply voltages and upper window voltage, but also to voltagechanges due to variations in the resistances of any or all the resistorsR, to R The slope of the tangent line plotted in FIG. 4 is directlyrelated to the stability of the SCAs. As the slope increases, thestability decreases for both of the cases under observation. As theslope approaches zero, the stability would approach infinity. In theexample shown, the slope would be about 8.7556 counts/min.- channel. Theequation for this line is approximately as shown, i.e.,y==4076.5-8.75576x. By recognizing that the spectrum window widths issmall compared to the width of the valleys shown in FIG. 4, the tangentline can, as an approximation, be used to represent the actual spectrum.

The number of counts in any given segment on the FIG. 4 graph can bedetermined by integrating the area under the tangent line between thesegment limits and multiplying the answer by the count interval. Thus,for segments 1 and 2 of FIG. 4 in a 2 minute count interval, the resultswould be: (l) Counts in segment 00 l= (4076 5-8 756x)clx which equals1,041,481 counts; (2) Counts in segment 242 2=2 (4076.5-8.756x) dx whichequals 571,972 counts. 1f the sulfur content in coal is determined bythe formula Y= /2 counts in seq. l)/(counts in segment 2) then the idealcase would yield a Tr value of 0.910430.

If the lower threshold voltage of SCA l was to shift to cause a decreaseof /2 channel while its window width remained constant, then the countsin segment 1 would be 1,043,381. Assuming SCA 11 and the segment 2values remain constant (i.e., 571,972 counts), then this V2 channelshift in one SCA results in an I of 0.912091 or an error of 1661 p.units from the ideal value of 0.910430. Now if our invention is used toswitch SCA l and SCA ll such that each of their outputs alternatelyoccupies segment 1 and 2, a comparison of the errors can be made. Withour invention, the count interval would be one minute from each of theSCAs for each segment, thus:

299.5 300 Counts in segment 1-]; 4276.58.756x)dxt+f (4076.5 8.756x)1,042,431 and as 242 4 counts in segment 2 =f(4076.5-8.756x)dxt+ 241.24{(4076.5-8.75x)dx 572,744. Hence,

Hence, 2 is ['k (l,O42,431)]/572,744 or 0.910032. This is an error of398 p. units (0.910430 0.910032). Comparing this to the error of 1,661units when there is no switching, the result is an error reduction ofafactor of4. l 7 1661/398) when the error is due to relative thresholddrift.

The type of error causing a change in the window width of one of theSCAs will be considered next. If the window width of SCA I decreases by/2 channel while the lower threshold remains constant and the SCA ofsegment 2 also remains constant then:

Counts in Segment 1 (4076.5-8.756x)dx 1,040,029

could be used to replace a multiple channel pulse height analyzer and asmall computer in a simpler and less costly way. Other variations arealso apparent as a person skilled in the art should realize. None ofthese variations should be used to limit the scope and extent of ourinvention which is to be measured only by the scope of the claims thatfollow.

We claim:

1. A method for stabilizing the analog output signals from two singlechannel analyzers that are to be fed to a comparator comprising thesteps of:

counts detecting an analog signal representative of some physicalphenomenon;

amplifying said representative signal and transmitting the result to amultiplexer system;

splitting said amplified signal in the multiplexer system into twoanalog signals and sending each signal to one of two separate singlechannel analyzers that output fixed pulses and normally operate betweenCounts in segment 1 4076.3-8.756x)da. f84076.5-8.756xldx 1,040,755counts over a 2 minute count interval.

For segment 2, the window width will be decreased one-half the time bythe same percentage as above to give equal window widths of l 16channels and 1 15,733.

42 24 Counts in segment 2 =fl4076.5-8.756x)dx 1(4076.5-8.756x)dz 571,449count This means f /2 (l,040,755)]/57l,449 0.910628.

Comparing this to the ideal value of 0.910430, we see that there is anerror of +198 p. units. Our invention would, thus, reduce the error dueto the window width changes, a factor of 6.4 (1,269/198) under theconditions stated.

It is readily understood from the above examples that the errors whichresult from changes in the reference voltages (B, through E thereference network resistors (R, through R and the offset voltages of thecomparators in SCA l and SCA 11 are transferred nearly equally into eachoutput channel by the time sharing of these circuits and voltages. Theoverall effect of time sharing then is to produce a change in the countof segment 2 that is proportional to any change (error) in the countinduced in segment 1 due to variations in voltages and circuitparameters, and visa-versa. Thus, in the formula for sulfur, the errorsin each count tend to cancel. That is, since the error count occurringin segment 1 is proportional to the count in segment 2 and the error inthe count of segment 2 is equally proportional to that count in segment2 then in the quotient setment 1 count divided by segment 2 count)cancellation of the errors occur.

It should be apparent that our invention is not limited to the specificsystem discussed'in FIGS. 1 to 4, but may be used in any systemrequiring a comparison of the counts in two areas of a count spectrumthat requires SCAs witha high degree of relative stability. It

same gate output; and

comparing the outputs from the gating circuitry after switching andgating takes place to obtain the desired results.

2. The method of claim 1 wherein said detection step detects radiateenergy from a radioactive source that is transmitted through a materialunder investigation.

3. The method of claim 1 wherein said switching step is accomplished byactuating a flip flop and gating means by a clock pulse.

4. A stable output multiplexed single channel analyzer systemcomprising:

means for receiving an analog signal representative of a physicalphenomenon;

two single channel analyzers operable within different specific rangesto receive said analog signal and to output fixed pulse signals whenoperative;

a multiplexer system to time share by switching and gating the outputsignals from said analyzers such that the output signals from theanalyzers are continuously switched and the same range always appears atthe same gate output; and

a comparator to compare the multiplexed signals outputted from themultiplexer system.

5. The system of claim 4 wherein said multiplexer system comprises:

a clock to generate a pulse;

a flip flop connected to said clock; and

a plurality of gating elements operatively associated with said singlechannel analyzer and said flip flop to alternately switch the outputpulses from the single channel analyzers.

6. A system for accurately determining the sulfur content of coalcomprising:

a radioactive ray source to transmit neutrons through coal;

a gamma ray detector to detect the created gamma rays transmitted andconvert these detected rays into electrical representations;

circuitry to amplify the electrical representations;

two substantially identical single channel analyzers normally operablewithin different specific ranges to receive said electricalrepresentations;

a multiplexer system connected to the outputs of said two substantiallyidentical single channel analyzers to analyze the electricalrepresentations from said analyzers between certain specific ranges,said system including switching and gating means to continuously switchthe analyzed signals from the analyzers and to gate these switchedsignals so that the signals from the same specific ranges always appearat the same gate output; and

comparator and scalar circuitry to compare the switched and gatedoutputs with each other and to output a scaled answer of their ratio.

7. The system of claim 6 wherein said multiplexer system switching meanshas a plurality of gating members actuated by drivers.

8. The system of claim 7 wherein said drivers are switched by a clockpulse actuated flip flop.

1. A method for stabilizing the analog output signals from two singlechannel analyzers that are to be fed to a comparator comprising thesteps of: detecting an analog signal representative of some physicalphenomenon; amplifying said representative signal and transmitting theresult to a multiplexer system; splitting said amplified signal in themultiplexer system into two analog signals and sending each signal toone of two separate single channel analyzers that output fixed pulsesand normally operate between different specific output ranges; switchingand gating the outputs from said single channel analyzers so that eachanalyzer alternately operates at one of two outputs with the samespecific range output signals always appearing at the same gate output;and comparing the outputs from the gating circuitry after switching andgating takes place to obtain the desired results.
 2. The method of claim1 wherein said detection step detects radiate energy from a radioactivesource that is transmitted through a material under investigation. 3.The method of claim 1 wherein said switching step is accomplished byactuating a flip flop and gating means by a clock pulse.
 4. A stableoutput multiplexed single channel analyzer system comprising: means forreceiving an analog signal representative of a physical phenomenon; twosingle channel analyzers operable within different specific ranges toreceive said analog signal and to output fixed pulse signals whenoperative; a multiplexer system to time share by switching and gatingthe output signals from said analyzers such that the output signals fromthe analyzers are continuously switched and the same range alwaysappears at the same gate output; and a comparator to compare themultiplexed signals outputted from the multiplexer system.
 5. The systemof claim 4 wherein said multiplexer system comprises: a clock togenerate a pulse; a flip flop connected to said clock; and a pluralityof gating elements operatively associated with said single channelanalyzer and said flip flop to alternately switch the output pulses fromthe single channel analyzers.
 6. A system for accurately determining thesulfur content of coal comprising: a radioactive ray source to transmitneutrons through coal; a gamma ray detector to detect the created gammarays transmitted and convert these detected rays into electricalrepresentations; circuitry to amplify the electrical representations;two substantially identical single channel analyzers normally operablewithin different specific ranges to receive said electricalrepresentations; a multiplexer system connected to the outputs of saidtwo substantially identical single channel analyzers to analyze theelectrical representations from said analyzers between certain specificranges, said system including switching and gating means to continuouslyswitch the analyzed signals from the analyzers and to gate theseswitched signals so that the signals from the same specific rangesalways appear at the same gate output; and comparator and scalarcircuitry to compare the switched and gated outputs with each other andto output a scaled answer of their ratio.
 7. The system of claim 6wherein said multiplexer system switching means has a plurality ofgating members actuated by drivers.
 8. The system of claim 7 whereinsaid drivers are switched by a clock pulse actuated flip flop.